AHTICULATA. 125 



The Crustacea constitute the second form or class of articulated 

 animals. They are provided with articulated and more or less com- 

 plexed limbs, attached to the sides of the body. Their blood is 

 white : it circulates by means of a fleshy ventricle placed in the 

 back, which receives it from the branchiae, situated on the sides of 

 the body, or under its posterior portion, and to which it returns by a 

 ventral and sometimes double canal. In the last or lower species, 

 the heart or dorsal ventricle is itself extended into a tube. They all 

 have antennae or articulated filaments, inserted in the fore-part of the 

 head, usually four in number, several transverse jaws, and two com- 

 pound eyes. A distinct ear is only to be found in some species. 



The Arachnides form the third class of the Articulata. Their 

 head and thorax, as in many of the Crustacea, are united in one 

 single piece, furnished, on each side, with articulated limbs ; but their 

 principal viscera are enclosed in an abdomen connected to the 

 posterior portion of that thorax. Their mouth is armed with jaws, 

 and their head furnished with simple eyes, that vary as to number, 

 but the antennae are always wanting. Their circulation is effected 

 by a dorsal vessel, which gives off arterial branches, and receives 

 venous ones from them ; but their mode of respiration varies, some of 

 them still having true pulmonary organs, which open on the sides of 

 the abdomen, while others, receive air by tracheae, like Insects. In 

 both of them, however, we observe lateral openings or true stig- 

 mata. 



The Insecta constitute the fourth class of the Articulata, and the 

 most numerous of all the animal kingdom. With the exception of 

 some genera, the Myriapoda, in which the body is divided into nu- 

 merous and nearly equal parts, it is always divided into three portions: 

 the head, furnished with the antennae, eyes and movith; the thorax, 

 to which are appended the feet and wings, when they exist; and the 

 abdomen, which is suspended behind the thorax and contains the 

 principal viscera. Those which have wings, only receive them at a 

 certain age, and frequently pass through two more or less different 

 forms before they assume that of the winged insect. In all their 

 states they respire by tracheae ; that is, by elastic vessels which receive 

 air through stigmata pierced on their sides, and distribute it by 

 infinite ramifications to every part of the body. A vestige of a heart 

 only is perceptible, consisting of a dorsal vessel, which experiences an 

 alternate contraction and dilatation, but to which, no branch has ever 

 been discovered, so that we are forced to believe that nutrition is 

 effected in this class of animals by imbibition. It is, probably, this sort 

 of nutrition which necessitated the kind of respiration proper to In- 



